Patent GB-549257-A (GB)
Title: Improvements in and relating to protective apparatus for electric circuits
IPC: H02H3/40
Filed: 1941-09-08
Abstract: 549,257. Protective cut-out systems. BRITISH THOMSON-HOUSTON CO., Ltd. (International General Electric Co., Inc.). Sept. 8, 1941, No. 11444. [A Specification was laid open to inspection under Sect. 91 of the Acts. March 7, 1942] [Class 38 (v)] In a reactance-type protective system depending on the fact that at the zero of the current cycle the reactance is proportional to e/(di/dt), where e, i are the instantaneous systems voltage and current, respectively, the means for deriving from the circuit a voltage proportional to di/dt is so constructed and arranged as to produce it only substantially at the moments of current-zero, the means preferably comprising a peaking transformer as in Specification 548,931. Voltages proportional to line voltages are impressed through a transformer 13 and full-wave rectifiers 16 ... 18 on to resistances 19 ... 21. The line currents are impressed on peaking transformers 23 ... 25 so as to produce in the secondaries 26 ... 28 a series of voltage-peaks O, Fig. 3, proportional to di/dt and occurring at the current-zero. this voltage being rectified at 29 ... 31 and impressed on resistances 32 ... 34. The voltages in each pair of resistances are in opposition, that due to the system voltage being in the restraining sense. The net voltage of each pair of resistances is fed through a unidirectional conducting device 39, 41, 43 to a relay 38 of the flux-shifting kind which is tripped by flow of current in one direction only, namely the operating direction, for example, a relay as described in Specification 500,472, but in a modification, Fig. 2 (not shown), an electronic relay is employed. Unidirectional conducting devices 40, 42, 44 are provided for bypassing restraining currents from the reversed-voltage rectifiers, and a glow-discharge valve 45 protects the relay 38 from over-voltage. The valve 45 and devices 39, 41, 43 are differently arranged when the devices 40, 42, 44 are omitted. The theory operation is shown by the curves in Fig. 3 in which the peaks O occur at the zero points of the current wave i, which also represents the resistance-drop Ri in the circuit assuming unit resistance. The curve di/dt leads the. curve i by 90 degress and for unit reactance represents the reactance drop Xi. The curve of impedance drop Zi is equal to the sum of Ri and Xi and this represents the system-voltage on fault, appearing rectified across resistances 19 ... 21 as shown by curve R<SP>1</SP>. When the peaks O, which are proportional to di/dt. are greater than the value of R' at the moment of comparison, namely, current zero, the relay 38 operates. Also at the moment of comparison, the resistance-drop is zero so that the arrangement is reactance-responsive only, even when the resistance of arc faults changes and in asymmetric current conditions. The output of the peaking transformers can be applied with rectification to the resistances since only the positive peaks O can produce operation, but the comparison is then made only once per cycle. Fig. 7 shows a modification having directional characteristics, so avoiding the need. for a starting unit and contacts 54 of Fig. 1. The output of transformer 13 is fed to the primaries 67 of transformers having their secondaries connected through rectifier valves 70 to one end of a resistance 71, 72, or 73. The opposite end of each resistance is connected to an intermediate tap 74 away from the mid point so as to produce unequal half waves R1, R2, Fig. 9, those R1 being made flat-topped as at R1a by means of glow-discharge valves 75 so as to avoid high peak values. Valves 76 ... 78 similarly restrict the peak output of the peaking transformer 23 ... 25 to a value less than that of peaks R1a so that operation of relay 38 is definitely blocked for those half-cycles and can only occur during the half-cycles when the peaks exceed R2. This latter arrangement is shown in Fig. 9, in which, however, the restraining voltages R1a, R2 are shown above the line for purposes of comparison, but if the direction of power flow is reversed, the peak which would overcome R2 is in the restraining direction and the operating peak cannot, as already stated, exceed R1a. In both Figures the relay 38 is restored by a solenoid 55 energized by closure of back-contacts 58 of the breaker.